In this study, we have demonstrated that acquisition of acute HCV in HIV-infected subjects is associated with changes in cerebral function measures using a cognitive assessment and cerebral 1H-MRS. We did not however, demonstrate significant differences in PK11195 BP when using PET imaging as an in vivo marker of microglial cell activation.
Acute HCV was independently associated with poorer executive function performance in this study, despite the relatively well-preserved CD4+ cell counts and young age of subjects enrolled. Importantly, these differences remained statistically significant after adjustment for the lower proportion of aHCV
subjects receiving cART. While this form of higher cognitive processing has previously been described in HIV and chronic HCV/HIV coinfection 
, it has not previously been associated with the acute phase of HCV. These findings suggest that a neurological disturbance involving the prefrontal cortex and fronto-striatal regions may occur, in a similar manner to chronic HIV infection 
. Aside from nadir CD4+ cell count, no association with executive function and other clinical parameters were found as an alternative explanation, supporting acute HCV viraemia as the potential cause of this deficit. Unfortunately, without cognitive assessment data which precedes HCV acquisition, it is not possible to confirm these findings reflect an acute deterioration in cognitive performance associated with the acquisition of HCV infection.
Statistically significantly greater mI/Cr ratios were observed in the BG of subjects with acute HCV, when compared to matched controls in our multivariate model. Proportions of subjects receiving cART were very similar in both groups. mI is an osmosensitive glial marker and plays a crucial role in cell volume regulation 
. The changes to mI/Cr ratio that were observed may represent increased neuroinflammation and glial proliferation during the early phase of HCV. Forton et al
have previously described increases of mI/Cr ratio in patients with chronic HCV, in the frontal white matter, rather than the BG. As the BG has a higher blood flow per unit volume 
than other cerebral locations, it is possible that greater and earlier exposure of this cerebral location may explain why the changes observed in our study during acute infection had not yet evolved in other areas of the brain. Furthermore, BG dysfunction is associated with symptoms of fatigue and inertia in other neurological disorders and therefore inflammatory changes in this location, may explain the cognitive deficits we observed 
. Interestingly and in contrast, in our earlier pilot study, we reported reductions in BG mI/Cr ratios in 10 subjects with acute HCV 
. The changes we now report within a larger cohort may reflect a time dependent phenomena whereby dynamic changes in mI/Cr ratios are occurring during the early course of acute HCV infection. In our previous work, an average of 16 weeks had elapsed since most recent negative HCV RNA result, suggesting very recent acquisition of HCV virus. In this larger MRS study, the mean time elapsed since last negative HCV RNA result was 24 weeks and therefore we postulate these osmosensitive glial markers may change rapidly during acute HCV infection. Alternatively, other factors such as the small size of the earlier pilot study or differences in HIV and cART parameters of the participants may be responsible for these apparently contradictory findings.
No differences in PK11195 BP were observed between our study groups. Several plausible explanations for these findings exist. First, microglial cells may not be activated by HCV in vivo
, but an alternative cell-type may be responsible, or indeed, CNS neurological disturbance may not occur at all. Alternatively, microglial cell recruitment may only take place during later stages of chronic HCV infection, as the inflammatory process may take time to develop, given the chronic nature of HCV infection over many decades. The first theory is perhaps refuted and last theory supported by the findings of Grover and colleagues who describe increased PK11195 BP, interpreted as microglial cell activation, in patients with chronic HCV without significant liver disease and by the findings of Cagnin and colleagues who observed similarly increased PK11195 BP in a small number of subjects with advanced, chronic HCV infection using the same PET methodologies 
. It is therefore feasible that during the acute period of infection with HCV, there is no change to levels of microglial cell activation. It is known that microglia are recruited from circulating macrophage/monocytes and animal model data have demonstrated that this process may take several months following cerebral insult 
. The mean time elapsed since negative HCV RNA test was 21 weeks in the 8 aHCV
subjects of this PET study, which is likely to be an over-estimation of actual
time since viral transmission. It is therefore possible, if our study was repeated several months later, different results would be observed. Another explanation for our findings is that due to ligand-factors including high plasma-protein binding, non-specific binding or low brain uptake, PK11195 as a neurotracer was insufficiently sensitive to detect subtle changes in microglial activation in such neurologically asymptomatic subjects, and that use of other, newly available, alterative ligands including 11
C PBR-28 
C DPA-713 
C DAA-1106 
may alter our findings. Further PET studies are needed to answer these questions. If microglial activation is a feature of the chronic phase of HCV infection, we postulate that the effects of circulating cytokines (rather than microglial cell activation) may be responsible for the cerebral function differences we observed in this study. The neuropsychological effects of endogenous circulating cytokines and chemokines are observed in the presence of chronic infection and are very similar to the CNS effects of therapeutically-administered interferon.
Interestingly, we observed an association between those receiving cART and lower PK11195 BP values in some cerebral locations, which has not been previously described and which suggests lower levels of microglial cell activation in subjects receiving cART, irrespective of acute HCV status. While only small numbers of patients were sampled and few number of subjects were not receiving cART, this suggests that, similar to systemic levels of inflammation, neuroinflammation may also be reduced in effectively treated HIV-infected subjects. Over time, ongoing inflammation within the CNS may result in progressive neuronal damage with subsequent clinical sequelae and these data suggest an additional potential benefit from cART, which although may be postulated, have not to date been clearly demonstrated.
Associations between recreational drug use, concurrent sexually-transmitted infections and acquisition of acute HCV have been described which may influence cerebral metabolite ratios and have a negative impact upon cognitive performance 
. For this reason, any subject with recent recreational drug misuse (within past 4 weeks) or current sexually transmitted infection known to cause CNS disease (such as early syphilis) were excluded. It remains possible, however, that high rates of historical drug misuse (with resultant cerebral damage) may have contributed to our observations. A further limitation of this work is the absence of longitudinal follow-up studies to identify if any increase of 11
C PK11195 BP evolves with duration of HCV coinfection, although it should be noted that ethical considerations on radiation dosage has precluded the evaluation of such longitudinal PET studies to date. Finally, the sample size for the 1
H-MRS and PET imaging studies, limits our ability to detect subtle differences and associations. PET scans with PK11195 are too costly for large-scale studies, but our work is of similar size to previous physiological PET studies and larger than other previously published work in this area.
In summary, we report changes to cerebral function measures in subjects with acute HCV, but have not found differences of PK 11195 BP, representing microglial cell activation, in the early months of HCV viraemia. If microglial activation is a late feature of chronic HCV infection, then there may be an additional clinical reason to treat and eliminate HCV in the acute phase where possible, as therapy-response rates are higher and recommended treatment courses shorter